Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise-modulated thermal energy. Thermography is concerned with materials which are not photosensitive, but are sensitive to heat or thermosensitive and wherein imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material, by a chemical or a physical process which changes the optical density.
Most of the direct thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
In direct thermal printing, the heating of the thermographic recording material may be originating from image signals which are converted to electric pulses and then through a driver circuit selectively transferred to a thermal print head. The thermal print head consists of microscopic heat resistor elements, which convert the electrical energy into heat via the Joule effect. The electric pulses thus converted into thermal signals manifest themselves as heat transferred to the surface of the thermographic material, e.g. paper, wherein the chemical reaction resulting in colour development takes place. This principle is described in “Handbook of Imaging Materials” (edited by Arthur S. Diamond—Diamond Research Corporation—Ventura, Calif., printed by Marcel Dekker, Inc. 270 Madison Avenue, New York, ed. 1991, p. 498–499).
A particular interesting direct thermal imaging element uses an organic silver salt in combination with a reducing agent. An image can be obtained with such a material because under influence of heat the silver salt is developed to metallic silver.
A thermal impact printer uses thus heat generated in resistor elements to produce in a certain image forming material, a localised temperature rise at a certain point, which, when driven high enough above a threshold temperature and being kept a certain time above this threshold temperature, gives a visual pixel. In practice, many pixels are being formed in parallel on a same line and then repeated on a line by line basis where the thermographic medium is moved each time over a small position.
The application of thermal heads is evolving more and more towards high resolution schemes. In the early years, thermal heads had low resolutions only (120 dpi), but starting from the early 80's, new technological inventions have driven this resolution into the 600 dpi area (e.g. U.S. Pat. No. 4,360,818 or 5,702,188). Unfortunately, this technology always puts some constraints on the electrical configuration and the controllability of the individual nibs. This comes from the fact that in most cases the construction is based on a screen printing technology which has limited resolution but gives a low cost and fast manufacturing benefit. Constrained by this limited resolution, special configurations are being used to increase the printing resolution of the thermal head despite some electrical inconveniences:    not all nibs are addressable at the same time. For this purpose, a switching in the supply voltage system must be performed. Neighbouring nibs in fact use partly the same switch for controlling the on/off state. Selection of the neighbouring nib is done using the power supply system.    not printing a pixel with a nib during an active time slice will still generate power in the nib, being of course much lower than the power of an activated nib.
Normally, this “time multiplexing” of control electronics in such a head will only lower the printing speed as not all nibs can be excited simultaneously and accordingly, this groups of nibs must be printed one after the other in time. This is illustrated in FIG. 1 based on U.S. Pat. No. 5,702,188. Here, every 2 nibs will have a common switch Si to the ground potential, effectively having 1 electronics switch Si for controlling two adjacent nibs. Selection of the left or right nib sharing a same switch Si is done by taking appropriate values of the voltages Va and Vb. In this case, a total line can only be printed using two print jobs controlling each time the same electronic switches but having a different set of supply voltages in the two cases. This way of controlling the thermal head will be denoted in the present invention disclosure by “a sub line printing method”. In each sub line, a specific group or set of heater elements or nibs are being addressed and the combination of all sub lines produces a full graphical line, having addressed all the heater elements over the full printing range of the print head.
The method of using “time multiplexing” for printing a full pixel line has some consequences on the graphical output because of two reasons: film movement and thermal coupling.
The process of printing a pixel line in 2 or more time frames will increase the length of the total time for printing a line. The transport of the graphical medium is normally of such a kind that medium transport will occur outside the time frame when the actual pixel printing happens. But this is only theory. The real movement of the graphical medium is rather complex because of the many mass-spring systems present in the system. For example mostly a rubber roller is used for pressing the medium against the nib line of the printer. This is a very elastic medium with distributed mass. The friction forces between the medium and the print head mostly also depend strongly on the thermal state of the nib line as the emulsion layer will undergo some hardness variations when heated up, this with the purpose of increasing diffusion processes inside the material for accelerating the image forming process. The drive system consisting of an electrical motor (reluctance based, PM based or mixed), belt systems, gears, . . . etc. also adds equivalent springs and inertia to the drive system. Because of the rapid acceleration and deceleration wanted regarding the medium transport, vibrations will be present on the transient phase of the movement. This means that when printing one group of pixels on the image forming material, it is not always guaranteed that the medium will be in exactly the same position when printing the next group of pixels. The more time is present between the printing of these 2 (or possibly even more) groups of pixels, the more chance one might have that vibrations on the medium transport will give a misalignment of the graphical output of these pixel groups. This will lead to Moiré effects in the graphical output and is not allowed.
Adjacent nibs are mostly thermally linked with each other. Heat transport from one nib to another occurs, mostly by conductive means, partly by radiative means. E.g. with reference to FIG. 1, when printing the A-pixels, a lot of heat will be transferred to the B-nibs, giving in practice a substantially increased graphical output depending on the thermal coupling between the A and B-nibs. Again, different pixel size between the several printed pixel groups may be found, giving again Moiré effects in the graphical output.
In a thick film head, the electrical resistance is formed by the deposition of a continuous track of a resistive conductive paste on a substrate, as shown in FIG. 2, e.g. using a screening technique. Electric contact fingers can already be present on this substrate or can be deposited later on the surface of the resistive nib line itself. Because of its construction, the nib track forms a continuous thermal structure without any barriers for heat inside. In fact, the individual nibs are formed by a delimitation of the electrical current configuration due to the location of the electrical contact fingers. But for heat, there is no delimitation, making that heat will always spread along the nib line when generated in one of the individual ‘nibs’. This is the ultimate reason for having cross-talk between neighbouring nibs and when printing a single line in several time frames. A control algorithm must determine for every nib of the thermographic print head the amount of energy that must be dissipated in the resistive element. Depending on the thermal construction of the thermal head, this can be a very simple controller, e.g. all nibs are isolated from each other, giving no visual interaction on the printed medium between the several pixels. But in practice, the controller algorithm must deal with a variety of real-world problems.
A first of such problems is the changing characteristics of the thermographic medium, giving different pixel sizes for a same nib energy, e.g. some examples:    a different physical thickness of the emulsion layer    a different chemical composition of the image forming components.
A second problem is formed by changing environmental characteristics like temperature and humidity:    a temperature rise of the environment must be taken into account as the image forming temperature will not rise as it is determined by the chemical composition of the emulsion layer    humidity changes the thermal capacity of the emulsion, producing different temperature rises when applying the same amount of energy.
A third problem is that the thermal process itself produces an excessive amount of heat which is not absorbed by the image forming medium. This excessive heat is absorbed by a heat sink, but nevertheless, gives rise to temperature gradients internally in the head, giving offset temperatures in the nibs and between the plurality of nibs. E.g. when the image forming process must have an accuracy of 1° C. in the image forming medium, an increased offset temperature of 5° C. in the heat generating element must be taken into account when calculating the power to be applied to that element.
A fourth problem is that the heat generating elements are in the ideal case fully thermally isolated from each other. In practice however, this is never the case and cross-talk between the plurality of nibs occurs. This cross-talk can be localised on several levels:    heat transfer between the plurality of nibs in the thermal head structure itself.    heat transfer in the emulsion and film layer itself.    pixels are not printed one aside the other, but partly do overlap on the print medium, mechanically mixing heat from one pixel with the other.
A further problem is that the electrical excitation of the nibs does mostly not happen on an isolated base. This means that not every nib resistor has its own electrical voltage supply which can be driven independent of all the other nibs. In general, some drive signals for driving the nibs are common to each other, this with the purpose of having reduced wiring and drive signals. In general, all nibs can be only switched on or off in the same time-frame. Producing different weighted excitations can only be achieved by dividing the excitation interval in several smaller intervals, where for every interval it can be decided whether the individual nib has to be switched on or off. This process of “slicing” has its influence on the thermal image forming process. For example: giving a pattern excitation with the weights (or driving times) (128,0,0,0,0,0,0,0) and (0,64,32,16,8,4,2,1) is mathematically only 1 point different, but the pixel size will be much more different than just 1 point in case of a commercial thermal head, because a ‘0’-no excitation interval produced in that specific device, produces heat in the nib as well! The controller has to take this effect into account.
In order to improve accuracy, the number of driving power levels has been increased, the nibs have got a higher resolution by decreasing the nib spacing, paper has been used which needs more heating or longer heating times, or which have a steeper characteristic (in order to increase pixel edge sharpness), but none of these solutions result in the improvement thought of, because a cross-talk problem comes in.
One way to counter-act on cross-talk is by making the active print period of each sub line, also called sub line time hereinafter, as short as possible. The longer it takes for a sub line to print, the more time is given to the heat to spread among the neighbouring nibs. Of course, a minimal time is present for each sub line, as the heater elements have a limit on the thermal power they can deliver and a minimum input power is necessary for the thermographic material to produce an image forming chemical reaction. The disadvantage of using a short sub line time is the fact that the controllability of the whole system is minimised, as there is no or little time left to produce numerous time slices, a technique necessary to control the power to the plurality of heater elements when being driven all by a common strobe signal (e.g. explained in EP-1234677). In practice, accurate control of the energy delivered to a heater element is mandatory, so as to compensate for shifted offset temperature in the heater element itself, the substrate carrying the heater element and parts of the heat sink. This shifted offset temperature is generated by latent heat present in parts of the print head because of printing activity in the past. As this latent heat depends strongly on the image information, a varying temperature profile can be found along the heater element zones in the print head and for accurate control, depending on the offset temperature in the heater element, an appropriate amount of energy must be delivered to the heater element in order to create equal size or equal dense pixels on the graphical medium. In practice, to avoid Moiré-effects in the graphical output and in order to obtain a uniform graphical output, independent of printing history, an accurate control on the temperature in the heater element is necessary and this accurate control should be independent of the location of the heater element. Using a time slice excitation scheme with a common strobe signal for driving all the heater elements, this individual heater element controllability can only be realised by taking numerous time slices, inevitably elongating the total time necessary to print a sub line.
However, using more time slices in a sub line, in favour of an increased controllability of the energy delivered to every heater element, does increase the total sub line time and, as a consequence, increases the cross-talk between the pixels being printed, as an elongated printing time allows the heat from one pixel to spread further to another one. This cross-talk inevitably generates Moiré-effects in the printout and puts bounds on the number of time slices that can be used in a sub line.
As an alternative to prevent Moiré effects, it is possible to increase the number of sub lines when printing a line and to introduce short waiting times between printing of different sub lines. Increasing the number of sub lines has the benefit of printing pixels more isolated from each other, making cross-talk more difficult by increasing the distance between nibs being active at the same instance of time. When having short waiting times between printing sub lines, the latent heat present in the nib structure has the time to spread and flow to the heat sink structure. This increase of the number of sub lines together with a good controllability of every sub line because of the presence of many time slices, allows to make high quality pictures. Unfortunately, this way the total line time will increase, giving, as a consequence, a lower graphical throughput of the printing device (measured in square meter/hour), something which is from an economical point of view mostly not acceptable. Therefore, one will mostly choose for a high material throughput of the printing device, despite the lower graphical quality of the printed material. Printing lines in two sub lines is known in industry with acceptable but unsatisfactory quality, and it is mostly used for screen making. No proposals for improvement of the image have been made, which is necessary if this method would be used for making film to illuminate. In that case, it must be possible to print e.g. 99% black, which is impossible at present.